1. Field Of The Invention
The present invention generally concerns amplitude modulation of radio frequency (RF) signals, and, more particularly, concerns very wide bandwidth amplitude modulation of the output signal of a high power RF amplifier or transmitter.
2. Background Of The Invention
The present invention deals with the modulation, or systematic alteration of one waveform called the carrier wave, according to the characteristics of another waveform called the modulating signal or message. The goal of the invention is to produce an information-bearing modulated wave whose properties are suited to a given communication task. One such communication task addressed by the present invention is solid state radar signal transmission, especially for military applications. The present invention also addresses the task of modulation of high powered signals transmitted within Quadrature Amplitude Modulated (QAM) communication systems. Both of these applications are characterized by requiring very wide bandwidth, ranging to hundreds of megahertz, amplitude modulation (AM) of microwave radio frequency (RF) signals exhibiting high power levels ranging to kilowatts and greater.
Amplitude modulation (AM) is well known in the art as a means of transmitting information upon a carrier signal wherein the amplitude of the carrier signal is modulated in response to the information signal. Amplitude modulation is commonly used for a number of applications including commercial radio transmissions. Amplitude modulators also have application in military systems in connection with high power and wide bandwidth systems, such as are utilized in radar systems or quadrature amplitude modulated (QAM) communications systems.
The present invention is specifically directed to performing linear, amplitude, modulation. As described below, the present invention effects linear AM exclusively by exponential, angle, non-linear modulation. Most particularly, the present invention affects AM by use of phase modulation (PM) which, along with frequency modulation (FM) is the major practical type of exponential, angle modulation.
Major differences between linear and exponential modulation in the spectrum of the modulated signal, in the bandwidth of the modulated signal, and in the signal-to-noise ratio of the modulated signal are well known. Those differences are often discussed in the context of the transmission of linearly versus exponentially modulated signals.
Contemporary transmission systems are typically unable to effectively perform amplitude modulation for high power RF signals (e.g. in the kilowatt range) having wide bandwidths (e.g. hundreds of MHz). Such contemporary systems typically generate an amplitude modulated RF signal at a low power level, nominally milliwatts, in a balanced mixer. A linear amplifier is then used to boost the amplitude modulated RF signal to high power levels. High power linear, Class A, amplifiers used in contemporary systems are relatively inefficient and expensive. Non-linear RF power amplifiers--particularly the Class C types--are much more efficient and inexpensive than their counterpart Class A types. Moreover, in practical forms, Class C amplifiers are generally capable of much higher output power levels. Class C type amplifiers cannot be substituted in contemporary transmission systems of the type discussed because they introduce unacceptable distortion into the amplified output signal. That distortion renders the information contained in the return signal substantially less discernible and therefore reduces the effectiveness of the system.
The present invention is directed to the production of a signal which is the exact equivalent of a linearly, amplitude modulated signal. In accordance with the present invention exponential modulation is used in an exponentially modulated signal to obtain a linearly, amplitude modulated signal permits the invention to advantageously utilize the capabilities, and efficiencies, of solid state (i) modulators and (ii) amplifiers of high power RF signals. Consequently, the present invention is concerned with the use of non-linear, nominally Class C, power amplifiers, i.e. in lieu of one or more linear amplifiers, to obtain a linearly, amplitude, modulated RF signal. Furthermore, as well as enabling the efficient and cost effective use of the preferred non-linear amplifiers, the present invention greatly extends (i) the bandwidth of modulation which may be effectively applied to RF signals and (ii) the power efficiency of the resulting amplitude modulated signal. Therefore, among other advantages, the present invention uses amplifiers preferred for their cost and efficiency, produces relatively fast rise times, allows realization of very wide bandwidth AM of RF signals, and allows this realization to be on RF signals which are of very high power.